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Lecture 3

lecture 3 & ch. 3

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Western University
Biology 2483A
Hugh Henry

Lecture 3 Chapter 3: The Biosphere  Warm/cold/dry/wet variations on earth – organism patterns because of that variation.  What the world is made up of biologically  Overview of the diff types of systems in the world and how they relate to the climatic factors in ch.2 The biosphere is the zone of life on Earth – found everywhere on earth. Biomes are large-scale biological communities shaped by the physical environment, particularly climate.  Biomes are categorized by dominant plant forms in that region - not taxonomic/relatedness o Tundra, boreal forest, grassland, ect. Why plants?  Dominant plants are visible, immobile, and occupy sites for a long time o Thus, they are good indicators of the physical environment, reflecting climatic conditions and disturbances that have occurred in that biome.  Because it can’t move, it must be able to tolerate all the conditions of that site during all times of the year  Terrestrial biomes are characterized by growth forms of the dominant plants, such as leaf deciduousness (shedding leaves) or succulence (ability to hold water - catcus) Plant growth forms Deciduous trees: loose their leaves during the cold/dry periods -south western ontario Needle-leaved evergreen trees: retain their photosynthetic tissues year-round - more as you move north Cacti & Shrubs: succulent steams and leaves contain water storage tissues Grasses & Sedges: nonwoody & grow from the base of their leaves Has good implications for grazing or fires, because it doesn’t affect the grass, just removes the leaves on top. Evergreen broad-leaved trees: in tropical regions carry out photosynthesis year round But do not loose their leaves like deciduous trees Scierophyllous shrubs: have tough, leathery leaves Forbs: board-leaved herbaceous (nonwoody & nongrass) plants Ex. Dandelions  Plants have taken many forms in response to selection pressures: such as aridity(dryness), extreme temperatures, intense solar radiation, grazing, and crowding.  Similar growth forms can be found on different continents, even though the plants are not genetically related. – similar adaptations will be in the same biome. o Convergence: Evolution of similar growth forms among distantly related species in response to similar selection pressures.  Temperature has direct physiological effects on plant growth form. o The interaction of precipitation and temperature act together to influence water availability and water loss by plants.  Water availability and soil temperature determine the supply of nutrients in the soil. Biomes Vary with Mean Annual Temperature and Precipitation  Problem: this does not account for seasonal variation which can have large effects (ex. due to tilt of the earth) Global Biome Distributions *memorize Global Biome Distributions Are Affected by Human Activities  Human activities influence the distribution of biomes. o Land use change: Conversion of land to agriculture, logging, resource extraction, urban development.  The potential and actual distributions of biomes are markedly very different – we’ve modified/effected many biomes A lot of the earth is ~40% changed from what it once was The parts of the world that haven’t been modified are the least desirable places – boreal, tundra, desserts, rainforests There are nine major terrestrial biomes.  Climate diagrams show the characteristic seasonal patterns of temperature and precipitation at a representative location. Tropical Rainforests WET & HOT - humid Mean temperatures every month is above 20 Stable high precipitation rates year round ~10% of earths surface & contain ~50% of Earth’s species. -- High biomass, high diversity—  Light is a key factor—plants must grow very tall above their neighbors or adjust to low light levels. – because its so humid, there’s a bunch of different mechanisms available: o Emergents rise above the canopy: adapt to grow very tall o Lianas (woody vines) and epiphytes use the other trees for support.  Normally growing on a tree would be a hostile environment in other systems, but its so moist that it’s not hindering here. o Understory trees grow in the shade of the canopy: adapt to low light conditions o shrubs and forbs occupy the forest floor.  This area is productive enough that there are many different layers of plants  Tropical rainforests are disappearing due to logging and conversion to pasture and croplands. o ~½ of the tropical rainforest biome has been altered.  Recovery of rainforests is uncertain: Soils are nutrient-poor, and recovery of nutrient supplies may take a very long time. o The soill is very old – never went through the last glaciation, so the soil is sucked dry of all its nutrients o So when you get rid of all the above ground plants – burn/clear them, it’ll take a long time to recover from that o Example of this: Fish-bone pattern caused by logging roads – can see an extensive clearing of rainforest over a short time period. Tropical Seasonal Forests and Savannas:  Located slightly N/S of equator o Wet and dry seasons associated with movement of the ITCZ.  Yellow on graph = drought/water stress  Still stable warm temperature  Different plants than in the rain forest: o Shorter trees o Strategies they use to deal with the dry seasons = deciduous - drop leaves in dry o See other strategies being more competitive: more grasses and shrubs. Tropical seasonal forests = more trees – more spares Tropical season savannas = more grasses, fewer trees How to convert a forest to a savanna:  Mostly due to climate factors, but also: o Fires promote establishment of savannas; some are set by humans.  Because grasses are more favorable/tolerant to fire and can grow back quicker o In Africa, large herbivores—wildebeests, zebras, elephants, and antelopes—also influence the balance of grass and trees – (by eating them) o On the Orinoco River floodplain, seasonal flooding promotes savannas.  Less than 1/2 of seasonal tropical forests and savannas remain. o Human population growth in this biome has had a major influence. o Large tracts have been converted to cropland and pasture. Hot Deserts:  High temperatures, low moisture.  Location: at high pressure systems – areas of subsidence  Yellow = drought o Low water availability constrains plant abundance and influences form. o Sparse vegetation and animal populations. o Cannot support a high biomass When it DOES occasionally rain, plants take advantage of this by growing, flowering, & seeding very quickly while it still remains moist Convergence in the Forms of Desert Plants Many plants have succulent stems that store water. There are such strong selection pressures in desserts that you see a lot of convergence Convergence of this form is shown by cacti (Western Hemisphere) and euphorbs (Eastern Hemisphere). - they look & act similar, but they are very distantly related.  Humans use deserts for agriculture and livestock grazing. – try to make them more productive o Agriculture depends on irrigation, and results in soil salinization.  Dessert soil have high [salt] b/c high evaporation and plants can’t handle that o Long-term droughts and unsustainable grazing can result in desertification—loss of all plant cover and soil erosion  Problem: in Africa the population depends on that agriculture – get famine Temperate Grasslands:  Great plains in Europe & North America  Warm, moist summers and cold, dry winters. o Blue graph = cold temp force adaption’s on these plants  Grasses dominate; maintained by frequent fires and large herbivores such as bison (historically) o Stop fires = more forests - Grasses grow from the base - to cope with dry conditions, more roots than stems and leaves o the deep root systems results in accumulation of organic matter and high soil fertility. - Most fertile grasslands of central North America and Eurasia have been converted to agriculture b/c rich soil  In arid grasslands, grazing by domesticated animals can exceed capacity for regrowth, leading to grassland degradation and desertification. o Irrigation in some areas causes salinization. Temperate Shrublands & Woodlands  Mediterranean life – Greece, Italy, Californi
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